This invention relates to optical filters and, in particular, to tunable all-pass optical filters.
Optical fiber communication systems are beginning to achieve great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, and optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Dispersion compensating devices are important components of optical communication systems. Chromatic dispersion occurs when signal components of different wavelengths are subject to different propagation delays. Such dispersion can distort a transmitted pulse and deteriorate the information content of a signal channel. Dispersion compensating devices equalize the propagation delays among the different wavelength components and maintain the quality of the transmitted information.
All-pass filters are useful in optical communication systems. An all-pass filter (APF) substantially equalizes phases among the different wavelength components of a signal with minimal modification of the amplitude response.
Accordingly an APF is highly useful in compensating chromatic dispersion. APFs are also useful in wavelength-dependent delay lines and in more complex filters.
Tunability is an important functionality in all-pass filters. Conditions in an optical communication system can change as channels are added, dropped and rerouted among branches. Consequently filters need to be tunable so that they can be adapted to changing conditions. Even in static applications tunability is useful to compensate fabrication variations.
FIG. 1 schematically illustrates a conventional tunable optical all-pass filter comprising An optical waveguide 10 coupled to a co-planar ring resonator 11 by two couplers 12A and 12B. The segment of waveguide 10 between the couplers and the adjacent portion of the resonator 11 form a Mach Zehnder interferometer 13. A first phase-shifter 15 in the waveguide and a second phase-shifter 16 in the resonator can be used to tune the filter.
In operation, a light pulse traveling in the waveguide 10 couples in part to the resonator 11. After transit around the resonator the light couples back to the waveguide. Interference between the light from the resonator and light transmitted on the waveguide produces a frequency dependent time delay that compensates dispersion. The response of the device is periodic in frequency, and the period is called the free spectral range (FSR).
The performance of the device depends on the resonator optical pathlength and the strength of coupling between the resonator and the waveguide. The resonator pathlength determines the FSR of the device, and the coupling strength determines the maximum group delay and the bandwidth of the delay.
Control over the phase-shifters 15, 16 permits tuning. These phase-shifters are typically localized heaters which change the refractive index of the underlying material. Control over phase-shifter 16 permits tuning of the resonator pathlength and hence the FSR. Control over phase-shifter 15 permits tuning of the phase difference between the waveguide arm and the resonator arm of the MZI. This tuning in turn, changes the coupling strength and thereby tunes the group delay and bandwidth.
These tunable filters work well for many applications, but with the demand for increasing bandwidth, smaller devices are required. But as the dimensions of the device become smaller, it becomes increasingly difficult to thermally isolate the waveguide and the resonator so that they may be independently tuned. Accordingly there is a need for a new architecture in tunable all-pass filters.
In accordance with the invention, a tunable optical all-pass filter comprises a substrate-supported multilayer waveguiding structure comprising a first layer including a waveguiding optical ring resonator, a second layer for spacing, and a third layer including a curved waveguide. The curved waveguide is optically coupled to the resonator by two spaced apart optical couplers extending through the spacing layer, and tunability is provided by a first phase-shifter to control the optical pathlength of the resonator and a second phase-shifter operative on the waveguide to control the strength of effective coupling between the input/output waveguide and the resonator. In one embodiment, the waveguide and the resonator are horizontally spaced apart in the non-coupling regions to provide optical isolation. In another, the waveguide and resonator can overlap horizontally, but the spacer layer is thicker in the non-coupling regions to provide optical isolation.